Devices for detecting reflected waves especially include sonars, radars and lidars. A device for imaging reflected waves comprises an emitter emitting wave trains, i.e. wave pulses. The wave trains or wave pulses are acoustic pulses in the case of sonars, and electromagnetic pulses in the case of radars and lidars. The emitter is arranged so as to emit at least one wave pulse toward an observed zone of the ground with a grazing incidence. These waves are reflected by the ground or objects on the ground and the imaging device comprises a receiver that measures the echoes reflected by the irradiated zone.
The ground may be located above water level in the case of radars and be the seabed in the case of sonars.
The invention is especially applicable to assistance with the detection of objects on the ground in images of the ground obtained by a side-looking device for imaging waves reflected from the ground. Such an imaging device is shown in FIG. 1. The imaging device 13 comprises an emitter 2 and a receiver 3 that are mounted on a carrier 4 moving above the ground 5 in a direction of advance D. The imaging device 13 is mounted on the carrier 4 so that the emitter 2 and the receiver 3 point, in a pointing direction Po, along a plane substantially perpendicular to the direction of advance D of the carrier. This, for example, is the case of side-scan sonars mounted on a carrier moving above the seabed, and of side-looking radars mounted on an aircraft moving above the ground.
The emitter 2 emits incident wave pulses at regular time intervals and the receiver has a directivity that is wide aperture in the vertical plane, i.e. in elevation, and very narrow in the horizontal plane, i.e. parallel to the direction of advance of the carrier, or its bearing. In other words, on each emitted wave pulse, the receiver forms an elementary channel measuring the echoes emitted by an elementary observed zone ZOi (i=1 to 3 in FIG. 1) that extends longitudinally along a direction contained in the same plane as the pointing direction Po beside the carrier. The emitter and receiver occupy substantially the same position, referred to as the position of the imaging device. In other words, they are arranged so that the emitter can intercept direct paths from the elementary observed zones. A direct path is a return wave following the same path as the incident wave that excited it. Each elementary observed zone ZOi extends widthwise parallel to the direction of advance of the carrier D. The elementary observation zone ZOi has a very thin width in the direction of advance of the carrier. Each elementary observed zone ZOi is the zone of the ground that is comprised in an elementary observation sector Si (only one of which S3 is shown in FIG. 1) having a narrow aperture in azimuth and a wide aperture in elevation. The echoes collected following the emission of an incident wave pulse and the reception, via an elementary reception channel of index i, represent the reflectivity of the bed along the elementary band in the elementary observed zone ZOi of index i.
Conventionally, devices for assisting with the detection of objects on the ground comprise a display device allowing an image representing the intensities of the echoes generated by the ground in an observed zone of the ground to be displayed, on a screen, at least along a distance axis d representing distances separating the imaging device from a reflector. An exemplary displayed image is shown in FIG. 2. The image 12 consists of a juxtaposition of elementary bands Bi that are also referred to as elementary channels Bi. The limits between the various bands Bi are represented by dotted lines in FIG. 2 because they are not visible in the images. Each elementary band Bi is a representation of the intensities of the echoes generated from an elementary observation zone ZOi under the effect of the incident pulse, which echoes are measured by forming a single elementary reception channel. Each channel represents the reflectivity of the ground between a minimum range distance dmin and a maximum range distance dmax of the imaging device. The elementary bands Bi extend longitudinally along the distance axis d, which is representative of the oblique distance separating the imaging device 2, 3 from echo-generating reflectors in the elementary observed zone ZOi during the formation of the associated elementary reception channel. FIG. 1 shows the oblique distance do separating the imaging device 13 from a point s of the ground in the sector S3. The image 12 is produced band-by-band with the movement of the vehicle. It is what is referred to as a “waterfall” image. Each band is acquired separately by the same periodic process consisting in emitting a wave pulse and in intercepting the echoes excited by this pulse.
Any echo detected in an elementary observed zone ZOi shows up as a bright spot in the corresponding elementary band of the image. The absence of echoes is represented by a dark zone on the screen. The background of the image is normally of intermediate intensity, since it represents the reverberation background but it is shown in white in FIG. 2 for greater clarity. A very advantageous effect of the emission of wave pulses with a grazing incidence is that projected shadows are formed in the image. If an object 6 on the ground 5 has a sufficient height h, it will emit echoes that will be displayed as a bright zone 7 on the screen (represented by dots in FIG. 2). Such an object intercepts a portion of the emitted wave, thereby preventing backscattering by the bed masked by the object beyond the object. The echo received by the receiver will be of a very low level over a certain duration, this showing up in the image as a dark zone 8 that is referred to as the image of the shadow projected in the image by the object 8 (represented by the hatched zone in FIG. 2). The shadow 8 projected by the object in the image may here be defined to be the image of that zone of the ground which the object 7 in question screens, preventing its irradiation by the incident wave pulse. This effect is highly advantageous in any application for detecting and classifying objects on the bed. This allows a noteworthy object to be located not by the echo that it reflects but by the shadow that it generates in the image. Shadow detection is particularly advantageous for locating certain objects, certain stealth submarine mines or stealth planes in particular, which reflect little or no radiation but that nevertheless screen and therefore produce a shadow. However, a drawback of shadow detection is that the size of the shadows in the images varies greatly, especially depending on the geography of the observed zone and the position of the imaging device relative to the observed zone. In other words, the variation in the size of the shadow of a noteworthy object having known dimensions as a function of its position in an image is not easily predictable by an operator, who encounters a certain number of difficulties detecting a noteworthy object in an image. By “detecting a noteworthy object in an image”, what is meant is identifying in the image the position of the image of an object having the dimensions of the noteworthy object.
Solutions for assisting an operator with detection of a noteworthy object on a screen do exist. A first type of solution consists of a tool allowing the operator to select the image of a noteworthy object on the screen and to designate the limits of the image of this object or of the image of its shadow on the screen. A processor then evaluates the dimensions of the associated noteworthy object, especially depending on the designated limits, on the resolution of the image and on the position of the shadow on the screen. This solution has the drawback of being very tedious for the operator and very time-consuming to implement since each object appearing in the image must be designated and its size must then be evaluated.
Another solution consists in using image-processing algorithms to detect the outlines of the images of shadows projected by objects and to evaluate the dimensions of the associated object. However, this solution exhibits a fairly low performance in terms of the probability of detection of images of shadows on the screen and in terms of high false alarm rates for noteworthy object detection (i.e. for the association of detected shadows with sought noteworthy objects). Specifically, if the ground is deformed or the image is polluted by multipath echoes (generated by low water height), the shadows are deformed or poorly contrasted. These shadows are then poorly detected or their dimensions are poorly evaluated by these automatic detection algorithms.
The aim of the invention is to mitigate the aforementioned drawbacks.